Spanish telecoms giant Telefonica said it expects to have 255 million customers by 2009 compared with 181 million at the end of 2005.
The comparative figure for 2005 included numbers from O2, the mobile phone operation bought by Telefonica last year.
The forecast included mobile, fixed, fixed broadband and pay TV clients.
O2 said its operating income ahead of depreciation and amortisation would rise between 7 and 11 per cent between 2005 and 2009 and that revenues at its operations in Britain, Germany and Ireland would increase between 6 and 10 per cent over the same period.
Telefonica said the number of mobile phone customers alone should be 186 million by 2009 compared with 99 million at the end of 2005, and ADSL customers should rise to 16 million from seven million over the same period.
The fixed-line phone business was likely to stagnate at around 48 million clients, Telefonica said.